Many specialized terms and abbreviations are used in the communications arts. At least some of the following are referred to within the text that follows, such as in this background and/or the description sections. Thus, the following terms and abbreviations are herewith defined:
3GPP 3rd Generation Partnership Program/Project
CCE Control Channel Element
HARQ Hybrid Automatic-Repeat-Request
LTE Long Term Evolution (e.g., of a 3G system)
OFDM Orthogonal Frequency Division Multiplexing
PBRI Pruned Bit-Reversal Interleaver
PCFICH Physical Control Format Indicator Channel
PDCCH Physical Downlink Control Channel
PHICHPhysical HARQ indicator Channel
QPP Quadratic Permutation Polynomials
WiMAX Worldwide Interoperability for Microwave Access
Electronic communication forms the backbone of today's information-oriented society. Electronic communications are transmitted over wireless or wired channels using electromagnetic radiation. The availability and capacity of electronic communications is typically limited by the bandwidth of the communications channel. Especially in wireless environments, the bandwidth of a communications channel may be limited by the finite nature of the electromagnetic spectrum.
The available bandwidth of a communications channel, even given a finite allocation of the electromagnetic spectrum, may be increased by adopting any of a number of different schemes. Certain schemes enable more information to be communicated in a given spectrum allocation. This efficient utilization of spectrum can reduce the cost of communication services being provided, can enable richer communication services to be provided, or both.
Example communication schemes include sharing spectrum in frequency, space, and/or time; compressing information; coding information; modulating data signals; combinations thereof, and so forth. Different communication paradigms rely on such communication schemes to varying degrees to efficiently utilize spectrum. An example of relatively modern communication paradigms involve those directed to OFDM systems. With OFDM systems, information blocks are allocated in both frequency and time. OFDM systems appear to offer relatively efficient utilization of spectrum for next generation communication systems.
FIG. 1 depicts a portion of an example spectrum resource grid 100 for an OFDM system. As illustrated, the horizontal dimension corresponds to frequency and the vertical dimension corresponds to time. The frequency dimension is divided into OFDM sub-carriers 104. The temporal dimension is divided into OFDM symbols 106. Spectrum resource grid 100 includes multiple resource elements 108.
Each resource element 108 is one OFDM symbol by one OFDM sub-carrier. In some OFDM systems, the smallest part of the electromagnetic spectrum that may be allocated is referred to as a resource block 102. In OFDM systems that are based on LTE, for example, a resource block 102 is typically twelve OFDM sub-carriers by seven (and sometimes six) OFDM symbols. It should be noted that a resource block 102 may have a different dimensionality. Also, the total number of available OFDM sub-carriers usually depends on a given system bandwidth.
Thus, in an LTE system for example, the structure of the OFDM signal contains resource elements 108 spaced in both time (OFDM symbols 106) and frequency (OFDM sub-carriers 104). These resource elements 108 are grouped into a collection of resource blocks 102 that make up the OFDM signal to be transmitted. Within this collection of resource blocks 102, certain resource elements 108 are designated to contain control channel signaling information.
In a cell-based wireless system, for example, base stations within each cell transmit these control channels to the various mobiles contained within the cells. Unfortunately, the transmissions from different cells potentially overlap in time and/or frequency, and they may interfere with each other when there is also spatial overlap. This interference may be particularly harmful because the control channel transmitted from a specific cell may be persistent to individual mobiles in other cells.
The control channel information is organized in a manner that makes it efficient to detect the essential information used to further decode both the control and data signals. The fields that are present in the control channel include the PHICH, PCFICH, and PDCCH fields. These fields are described in 3GPP, Technical Specifications 36.212 v8, “Multiplexing and Channel Coding (Release 8),” 2007. Certain example variables and OFDM characteristics that are described herein relate to an LTE implementation; however, the traits and principles that are described herein are applicable to other types of OFDM systems.
One factor relevant to transmitting the control channel signal is that the applicable control information be spread across frequency so that frequency diversity may be obtained. Because the control signal uses a fixed-rate coding (e.g., in LTE), frequency diversity is particularly pertinent to providing reliable detection of the control signal. Another relevant factor is that control channel transmissions may originate from multiple base stations, and their signals may therefore collide in a persistent manner. This potential state of persistent collision, coupled with a non-uniform setting of the transmit power, may result in persistent interference from neighboring base stations for some mobiles. Consequently, the control channel signal in LTE is to use some form of interference randomization to at least partially alleviate this interference.
One prior approach that has been proposed to address these issues is described in R1-074226, “Generic Interleaver for PDCCH,” Huawei, YSG RAN WG1 meeting #50bis, Shanghai, China, Oct. 8-12, 2007. This approach uses a common interleaver design to permute symbol groups, followed by a cell-specific cyclic shift to further distinguish the transmitted signals of different base stations. This basic approach, using a common interleaver followed by a cell-specific cyclic shift, is adopted in a number of cases (e.g., R1-073994, R1-074080, R1-074318, and R1-074370). Each of these cases does, however, describe a different interleaver design. In R1-074194 (“Downlink control signaling for SU-MIMO,” LG Electronics, YSG RAN WG1 meeting #50bis, Shanghai, China, Oct. 8-12, 2007.), a similar approach is taken to try to achieve diversity while addressing the interference. However, in R1-074194 a cell-specific interleaver is used instead of the common interleaver design.
Common aspects for each of the approaches mentioned above include the following considerations. First, the PHICH, PCFICH and PDCCH control information are collected into symbol groups of four subcarriers located relatively close together. This collection is called a mini-CCE. Second, a number of mini-CCEs form a CCE. CCEs are concatenated together to form the PDCCH. Third, the PDCCH mini-CCEs are interleaved, and then they are mapped to the resource elements. Fourth, the PHICH and PCFICH may be fixed within the OFDM subframe or interleaved together with the PDCCH. Fifth, the mapping takes place over first one, two, or three OFDM symbols continuously.
One example existing mapping approach has the mini-CCEs ordered by resource block. This existing mapping approach is described in PCT Patent Application No. PCT/SE2008/050372, which was filed 31 Mar. 2008 and entitled “Method and Arrangement in a Telecommunication System,” by inventors K. Molnar, J-F. Cheng and S. Parkvall for Applicant Telefonaktiebolaget LM Ericsson. PCT Patent Application No. PCT/SE2008/050372 claims priority from U.S. Provisional Patent Application No. 60/974,949, which was filed on 25 Sep., 2007. This resource-block-oriented mapping approach is shown in FIG. 2, where there are 8 mini-CCEs per resource block.
FIG. 2 illustrates an example of symbol groups defined and mapped continuously over three OFDM symbols in accordance with an existing approach. Mapping 200 is shown with OFDM sub-carriers along the horizontal axis and with OFDM symbols along the vertical axis. Mapping 200 includes one resource block 102 and the beginning of an adjacent resource block to its right. Each resource block includes at least one reference element 202. Resource block 102 includes four reference elements 202 as represented by the shaded blocks. Reference elements 202 are used, for example, for channel estimation but not for data or control channel transmission. Three OFDM symbols are shown for mapping 200 because three OFDM symbols are available for control channel information in this example. Hence, the other (e.g., four) OFDM symbols of resource block 102 are omitted.
As illustrated, each resource block 102 includes eight mini-CCEs numbered 1 to 8. Within a given resource block 102, the mini-CCEs are order by frequency first, then OFDM symbol, and lastly across resource blocks. This approach is described in PCT Patent Application No. PCT/SE2008/050372 in order to preserve frequency diversity when performing the PDCCH interleaving. In PCT Patent Application No. PCT/SE2008/050372, which is by the same inventors as the instant patent application, the use of a QPP interleaves is described because it has good frequency diversity properties. Other interleaving approaches include the approach proposed in R1-074226, which is based on the use of a Costas array. The Costas array is considered to have good autocorrelation properties, and it can provide good interference randomization.
A further difficulty arises in that the PHICH and PCFICH are expected to be detected prior to detecting the PDCCH, which may span one, two, or three OFDM symbols. Interleaving the PDCCH together with the PHICH and/or the PCFICH is undesirable inasmuch as it is then ambiguous as to exactly where the PHICH and PCFICH fields are located. One alternative is to fix the position of the PHICH and PCFICH fields so that they are located in known positions. However, if these two fields are fixed, then no interference randomization may be instituted to inoculate them from persistent interference.
Consequently, there is a need to address these deficiencies in the current state of the art. Such deficiencies and other needs are addressed by one or more of the various embodiments of the present invention.